Robust single trial identification of conscious percepts triggered by sensory events of variable saliency

Neural responses to sensory novelty with and without conscious access

Detection of novel stimuli that violate statistical regularities in the sensory scene is of paramount importance for the survival of biological organisms. Event-related potentials, phasic increases in pupil size, and evoked changes in oscillatory power have been proposed as markers of sensory novelty detection. However, how conscious access to novelty modulates these different brain responses is not well understood. Here, we studied the neural responses to sensory novelty in the auditory modality with and without conscious access. We identified individual thresholds for conscious auditory discrimination and presented to our participants sequences of tones, where the last stimulus could be another standard, a subthreshold target or a suprathreshold target. Participants were instructed to report whether the last tone of each sequence was the same or different from those preceding it. Results indicate that attentional orientation to behaviorally relevant stimuli and overt decision-making mechanisms, indexed by the P3 event-related response and reaction times, best predict whether a novel stimulus will be consciously accessed. Theta power and pupil size do not predict conscious access to novelty, but instead reflect information maintenance and unexpected sensory uncertainty. These results highlight the interplay between bottom-up and top-down mechanisms and how the brain weights neural responses to novelty and uncertainty during perception and goal-directed behavior.

Conscious Perception: Time for an Update?

Understanding the neural mechanisms underlying conscious perception has become a central endeavor in cognitive neuroscience. In theories of conscious perception, a stimulus gaining conscious access is usually considered as a discrete neuronal event to be characterized in time or space, sometimes referred to as a conscious “episode.” Surprisingly, the alternative hypothesis according to which conscious perception is a dynamic process has rarely been considered. Here, we discuss this hypothesis and its implications. We show how it can reconcile inconsistent empirical findings on the timing of the neural correlates of consciousness and make testable predictions. According to this hypothesis, a stimulus is consciously perceived for as long as it is recoded to fit an ongoing stream composed of all other perceived stimuli. We suggest that this “updating” process is governed by at least three factors (1) context, (2) stimulus saliency, and (3) observers' goals. Finally, this framework forces us to reconsider the typical distinction between conscious and unconscious information processing.

Neuroimaging somatosensory perception and masking

The specific cortical and subcortical regions involved in conscious perception and masking are uncertain. This study sought to identify brain areas involved in conscious perception of somatosensory stimuli during a masking task using functional magnetic resonance (fMRI) to contrast perceived vs. non-perceived targets. Electrical trains were delivered to the right index finger for targets and to the left index finger for masks. Target intensities were adjusted to compensate for threshold drift. Sham target trials were given in ~10% of the trials, and target stimuli without masks were delivered in one of the five runs (68 trials/run). When healthy dextral adult volunteers (n=15) perceived right hand targets, greater left- than right-cerebral activations were seen with similar patterns across the parietal cortex, thalamus, insula, claustrum, and midbrain. When targets were not perceived, left/right cerebral activations were similar overall. Directly comparing perceived vs. non-perceived stimuli with similar intensities in the masking task revealed predominate activations contralateral to masks. In contrast, activations were greater contralateral to perceived targets if no masks were given or if masks were given but target stimulus intensities were greater for perceived than non-perceived targets. The novel aspects of this study include: 1) imaging of cortical and subcortical activations in healthy humans related to somatosensory perception during a masking task, 2) activations in the human thalamus and midbrain related to perception of stimuli compared to matched non-perceived stimuli, and 3) similar left/right cerebral activation patterns across cortical, thalamic and midbrain structures suggesting interactions across all three levels during conscious perception in humans.

Objective measurement of visual resolution using the P300 to self-facial images

OBJECTIVE

To assess visual acuity objectively "beyond V1", the P300 event-related potential is a promising candidate and closely associated with conscious perception. However, the P300 can be willfully modulated, a disadvantage for objective visual acuity estimation. Faces are very salient stimuli and difficult to ignore. Here, we present a P300-type paradigm to assess visual acuity with faces.

METHODS

Gray-scale portraits of the respective subject served as oddball stimuli (probability 1/7), scrambled versions of these as the standard stimuli (probability 6/7). Furthermore, stimuli were spatially high-pass filtered (at 0, 2.2, 4.2 and 8.3 cpd), making them recognizable only with sufficient acuity. Acuity was systematically reduced by dioptric blur, chosen individually to render faces unrecognizable when high-passed at ≥ 4.2 cpd. EEG was recorded from 11 subjects at 32 scalp positions and re-referenced to the average of TP9 and TP10. One of the rare face variants was designated as target, for which a button had to be pressed.

RESULTS

The event-related potential was dominated by the P300 at 300-800 ms. All subjects had a significant (P < 0.05) P300 for 0- to 8.3-cpd filtering. When vision was blurred, the fraction of significant P300 responses to 8.3-cpd filtered faces dropped to 18%, but stayed at 100% for 4.2 cpd. Another component, the vertex positive potential (VPP) at 170 ms, was undetectable in most participants with blur and all levels of filtering, even when the images were recognizable.

CONCLUSIONS

The study demonstrates the feasibility of a face-based P300 approach to objectively assess visual acuity. The sensitivity to stimulus degradation was comparable to that of a grating-based approach as previously reported. An unexpected finding was the differing behavior of the P300 and the VPP. The VPP was quite sensitive to high-pass filtering, while the P300 sustained stronger filtering, although for its generation, the faces must also be discriminated from scrambled faces.

Neural signals evoked by stimuli of increasing social scene complexity are detectable at the single-trial level and right lateralized

Classification of neural signals at the single-trial level and the study of their relevance in affective and cognitive neuroscience are still in their infancy. Here we investigated the neurophysiological correlates of conditions of increasing social scene complexity using 3D human models as targets of attention, which may also be important in autism research. Challenging single-trial statistical classification of EEG neural signals was attempted for detection of oddball stimuli with increasing social scene complexity. Stimuli had an oddball structure and were as follows: 1) flashed schematic eyes, 2) simple 3D faces flashed between averted and non-averted gaze (only eye position changing), 3) simple 3D faces flashed between averted and non-averted gaze (head and eye position changing), 4) animated avatar alternated its gaze direction to the left and to the right (head and eye position), 5) environment with 4 animated avatars all of which change gaze and one of which is the target of attention. We found a late (> 300 ms) neurophysiological oddball correlate for all conditions irrespective of their complexity as assessed by repeated measures ANOVA. We attempted single-trial detection of this signal with automatic classifiers and obtained a significant balanced accuracy classification of around 79%, which is noteworthy given the amount of scene complexity. Lateralization analysis showed a specific right lateralization only for more complex realistic social scenes. In sum, complex ecological animations with social content elicit neurophysiological events which can be characterized even at the single-trial level. These signals are right lateralized. These finding paves the way for neuroscientific studies in affective neuroscience based on complex social scenes, and given the detectability at the single trial level this suggests the feasibility of brain computer interfaces that can be applied to social cognition disorders such as autism.